1. Field of the Invention
The present invention is directed to a controllable broad-band frequency generator comprising a conventional voltage-controlled oscillator (VCO), and a control loop, the control loop including a digital frequency discriminator and a crystal oscillator reference signal source.
2. Background of the Invention
Telemetry systems often require linear response, voltage-controlled frequency generators. Typically, a conventional voltage-controlled oscillator (VCO) is used for such purposes. However, in various applications, for example in space telemetry, a frequency generator is required not only to be substantially linear in response to applied control voltages over a wide band of frequencies but also to be stable in response for a long period of time. A typical example of such a space telemetry application requires a linear response over a deviation of .+-.7.5% from the center frequency with stable operation over seven years. Conventional VCOs are incapable of maintaining the above-mentioned requisite linearity and stability of response. For example, in space applications thermal variations, and radiation degradation affect the linearity and stability of a VCO response, in addition to changes in the VCO characteristics due to long term aging. Crystal oscillators maintain requisite stability of response, but are incapable of providing a linear response over a sufficiently wide bandwidth of frequencies. Thus, an engineering dilemma arises.
The prior art has approached the solution to the problem defined by such a dilemma by providing a conventional VCO in a feedback control loop for stabilizing the VCO output frequency response. The feedback loop includes a crystal oscillator frequency source, a frequency discriminator, a difference amplifier, and an integrator circuit. Such a system is disclosed in U.S. Pat. No. 3,753,141, by Van Elk et al., issued Aug. 14, 1973. The Van Elk et al. system, however, utilizes a frequency discriminator comprising digital counters and a D/A converter and is thus disadvantageous in that the control loop feedback voltage from the discriminator to the VCO is necessarily quantized, as opposed to being both continuous and linear. More specifically, the feedback signal generated by the discriminator must be in accordance with one of the possible discrete states of the counter and accordingly only approximates a straight line response by a step staircase. The number of steps in the staircase is determined by the number of bits in the D/A converter.
It is desirable to have a stable, wide-bandwidth, linear frequency generator wherein the feedback signal is continuous and linear, and which utilizes less components than that required in the prior art systems.